The invention relates to a sensor system located in a measurement casing according to the preamble of claim 1.
A comparable sensor system is e.g. known from EP 0 166 530 B1 or DE 37 02 838 A1.
The principle of these known sensor systems makes it possible to measure the oxygen partial pressure in a gas or in a gaseous mixture and in addition the moisture content of the gas can be determined.
These sensor systems have an oxygen-conducting plate for the individual sensor and which is in particular made from zirconia (ZrO.sub.2). When a voltage is applied to the electrodes on said plate the oxygen is pumped out of the interior of the diffusion casing. This pumping effect resulting from the applied voltage is greater than the quantity of the oxygen entering through the gas diffusion opening of the sensor. The current flowing through the zirconia plate is therefore dependent on the rate of the oxygen ions diffusing through the zirconia plate. However, as the oxygen flow quantity is limited due to the gas diffusion opening, there is initially a current saturation at a given voltage and in this phase the number of oxygen ions diffusing through the zirconia plate corresponds to the oxygen quantity entering through the gas diffusion opening.
When the aim is to establish the moisture content of a corresponding gas or air using such a sensor system, it must be borne in mind that primarily the oxygen partial pressure is reduced by the water content of the gas to be measured. Therefore the current saturation range is formed at a lower current intensity.
On further increasing the voltage between the electrodes on the zirconia plate, due to the diffusion of oxygen through the gas diffusion opening the current flow initially remains constant until, as a result of the higher voltage, the water vapour or steam decomposes into oxygen and hydrogen. This oxygen resulting from the dissociation of the steam now brings about an increase of the current flow. Thus, a further current saturation range forms, which in absolute values is above the current flow of the same gas without moisture.
Thus, for calculating the moisture content of a moist, oxygen-containing gas two saturation current ranges are obtained, whose current difference makes it possible to determine the moisture content in the gas to be measured.
In known sensor systems operating with the two aforementioned different current levels, an important disadvantage occurs in that on modulating the zirconia sensor to the two different voltage or current levels overshoots occur, so that no details on the corresponding moisture content can be obtained until there is a stable build-up.
It has also been found that the determination of an optimum modulation function is relatively difficult in the case of different moisture contents and consequently the known sensor systems suffer from a considerable time loss when calculating the moisture content.